I've been haunted by Anne Frank's memory for so long

Eva Schloss is Anne Frank's stepsister. She also lived in hiding and was eventually betrayed to the Nazis. But Eva survived Auschwitz and for three decades has worked to keep alive the memory of her half-sibling who perished in Bergen-Belsen

Eva Schloss, Holocaust survivor and stepsister of Anne Frank, at home in London. Photograph: Felix Clay for the Guardian

In the front room of her small London flat, Eva Schloss, 83, is recollecting her childhood and explaining how the events of those years coloured the rest of her life. An Austrian Jew whose family suffered at the hands of the Nazis during the second world war, she, her older brother and parents were sent to Auschwitz-Birkenau when she was 15, and only she and her mother came out again.

But while Eva is one of many survivors of the Holocaust, there is an element of her story that makes her unique: her mother later married Anne Frank's father, making her, to an endlessly curious world, the posthumous stepsister of one of the war's most famous casualties.

"He was a very kind, wonderful man, and a loving stepfather," Eva says of Otto Frank. "But he was very emotionally involved, still, with Anne and the preservation of her memory. Her presence became all-consuming in our lives."

This was, she admits, an occasionally suffocating obsession. "He talked about her continuously and, I must say, I did become a little jealous." As did her children decades later. "My daughters wanted to know why their grandfather was always talking about somebody else, just as I would get upset when I was introduced as her stepsister. I would say, 'I'm a person myself!' But I realised you cannot be resentful of someone who is no longer living."

Eva went on to have a full life and for four decades did not talk about her experiences in a concentration camp. It was only after Otto's death in 1980 that she felt compelled to take on the responsibility of keeping Anne Frank's name alive. She gave talks, visited schools and wrote books: Eva's Story in 1988, The Promise in 2006, which was aimed, like her stepsister's diary, at younger readers, and now After Auschwitz. "I'd written about life in the camps before, but nothing after that. It might sound ridiculous, but I found real life much more difficult. It took me a long time to find peace."

Though much of the new book dwells on her struggle to put the past behind her, she also addresses the stepsister she hardly knew in life. Born a month before Anne, the two girls shared a circle of friends from the ages of 11 to 13 in their adopted Amsterdam.

"If you are one of those millions who have read The Diary of Anne Frank," she writes in After Auschwitz, "you may believe that you already know a lot about her. I did not know this Anne Frank, of course."

The portrait she paints is of a precocious, self-confident girl, interested in boys, clothes, hairstyles and filmstars. Did she like her? "Not particularly. I was a tomboy and she was so much more sophisticated. We simply didn't have the same interests."

Then the war intervened and she and Anne never saw each other again. In 1942, her brother and father were ordered to report to a German "work camp", forcing Eva and her mother to go into hiding. For two years, they lived in terror of being discovered. It was in May 1944, shortly after moving into their seventh hiding place in Holland, that a Dutch nurse betrayed them to the Gestapo. They were arrested, tortured and sent to the Auschwitz-Birkenau concentration camp. "Life was full of horror and fear," she writes in After Auschwitz. "Imagine the hunger. Try to imagine the filth."

Eva Schloss with her mother, Fritzi, and older brother Heinz, who died in Auschwitz. Photograph: Copyright Eva Schloss Collection

Eight months later, with Germany facing defeat, they were released and began the long journey home.

In June, the war over, they reached Amsterdam. They met Otto Frank again, whose own family had also been destroyed. United in mourning, Eva's mother and Otto then began a life together, working in tandem to bring Anne's diary to publication.

"My mother and Otto had a very happy marriage. They were inseparable and that was sometimes difficult for me," Eva says. "I never got her to myself and couldn't understand, at first, how she could be so happy with someone who was not the father of her children. In many ways," she adds, "I think I got over my suffering at Auschwitz pretty quickly. But the loss of my family members, I was never quite able to accept, then or now."

She fell into a depression and at 16 became suicidal.

Anne Frank's The Diary of a Young Girl became a global sensation and gradually a curious sort of notoriety attached itself to Eva. Naturally, she read it, but wasn't greatly impressed. "I could see its appeal, though. In the 50s and 60s, people were starting to show interest in what had happened in the war, but didn't want to be reminded of the full horrors. Anne's book wasn't about the Holocaust at all. It was about hiding away. That was nothing new to me. I had hidden away in the war too before we were captured. But nobody wanted to hear my story."

By the time her mother and Otto Frank were married, in 1953, Eva had moved to London. She wanted to be a photographer (she would later run an antiques shop), and it was here that she met her husband, Zvi Schloss, a German Jew whose family had escaped internment by fleeing to Palestine.

In her new home, Eva was desperate to start a family and for a specific reason. When her brother Heinz was 12 he became terrified of dying. Their father explained that there was nothing to fear: when you have children, you live on through them. "But what if we don't live to have children?" he replied. Heinz died in Auschwitz.

Getting pregnant for Eva, then, was imperative; she needed to honour his memory. "But it was difficult. I still had mental and physical problems, and many difficulties conceiving."

When at last she did, "It brought me great happiness."

Eva had three daughters, now in their 50s, and she has five grandchildren. Does she feel she made a good mother. "That is actually a very sore point between my children and me," she answers. "I think I did, yes, and I did everything I could for them. I loved them very much."

But when, in a previous book, she wrote that she managed to have a normal life despite the horrors she had endured, her daughters questioned the veracity of this statement. Why? "You know, I don't know. I must ask them. I don't know what I must have been lacking … But the pain [of Auschwitz] was with me still, obviously."

The spectre of Anne Frank loomed large in her daughters' upbringing as well. It was Otto Frank who introduced her diary to them, underlining not just how important it was, but its personal significance to them. They all read it, Eva says, "but didn't talk to me about it so I never really knew how they felt". Did she ask? "No."

It's different with her grandchildren. She suggests that in many ways she has a better, or at least a less complicated, relationship with them than with her daughters. "Maybe because there is a bigger distance. But they want to know, they want to talk to me about it. They are interested. One of my granddaughters – she is 18, and studying German – wants to go into my family background a lot. So we talk about it much more openly."

Eva Schloss didn't choose this role – the books, the lectures, the talks she gives in schools for which she was awarded an MBE in 2012. Instead, the role was bequeathed to her – by her mother, and the man her mother married.

"I have two lives," she says. "In one, I go out and travel the world and speak about Anne. This is not something I discuss at home. At home, I'm simply a housewife, a mother, a grandmother."

After Auschwitz: My Memories of Otto and Anne Frank is published by Hodder & Stoughton on 11 April, £20. To order a copy for £15, including free UK p&p, go to guardian.co.uk/bookshop or call 0330 333 6846